Non-corrective eyewear includes safety glasses, sunglasses, or other non-corrective accessory glasses such as 3-D glasses. Corrective eyewear includes eyeglasses having corrective lenses worn to compensate for any of the following vision deficiencies: astigmatism, myopia, hyperopia, and presbyopia.
Astigmatism is caused by non-uniform curvature in the refractive surfaces of the eye resulting in distortion of both near and distant objects. Astigmatism is correctable with cylindrical lenses.
Myopia, or nearsightedness, occurs when the distance between the front and back of the eye is too long such that light is focused prior to reaching the retina, resulting in blurring of distant objects. Myopia is correctable with concave corrective lenses.
Hyperopia, or farsightedness, occurs when the distance between the front and back of the eye is too short such that light is focused beyond the retina, resulting in near-vision (or near-field) blurriness. Hyperopia may be compensated with convex lenses typically found in reading glasses or bifocal lenses.
Presbyopia is caused from aging and results in increased difficulty in near-vision focusing. Presbyopia, like hyperopia, may be corrected with convex lenses; however, unlike the first three forms of vision deficiencies, presbyopia, is presently not correctable by surgery because it is not directly related to the distance between the front refractive eye surface and the retina. Consequently, many older individuals eventually require reading glasses.
Reading glasses are generally available in two styles: full-frames and half-frames (half-glasses or half-eyes). Full-frames have full-sized corrective lenses, while half-frame styles have smaller half-lenses adapted for placement lower on the user's nose. Full-sized lenses provide a wider viewing angle (or field of view) for viewing close-up objects; however, the full-sized lenses also obstruct and blur distant objects. A person wearing full-frame reading glasses cannot readily observe distant objects without first removing the glasses. Conversely, the smaller half-lenses in a half-frame reduces interference with user's normal field of view, i.e., forward-facing or horizontal field of view, but these half-frame styles still obstruct peripheral vision while offering none of the wider viewing angle advantages. The half-frames are therefore less suitable for people requiring a wider near-field view for viewing close-up objects. Furthermore, the half-frames, or “Ben Franklin” styles, are potentially less aesthetically pleasing or less comfortable as they are typically worn lower on the nose.